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Featuring high electrical conductivity and electrochemical activity, transition metal sulfides are promising materials for the positive electrode of hybrid supercapacitors. However, considering their poor cycling life and rate performance, critical challenges remain. In this contribution, we describe hollow NiS2/CoS2@C composites obtained by stepwise etching, annealing, and sulfuration of ZIF-67. Specifically, NiS2/CoS2 particles were dispersed on N/S co-doped carbon nanocages and found to exhibit outstanding stability and rate performance. The unique nanocage structure of N/S co-doped carbon and heterogeneous interfaces of NiS2 and CoS2 can promote both ion transport and electron transfer, enriching the active site for enhanced reactivity. The resulting NiS2/CoS2@C nanocage composites in fact showed considerable electrochemical performance, with a specific capacity of 1373 C g-1 at 1 A g-1. When combined with reduced graphene oxide (RGO)-based negative electrode, the assembled hybrid supercapacitor registered an energy density of 63.3 Wh kg-1 at a power density of 800 W kg-1, retaining exceeding 98% of the initial capacity even after 13,000 cycles.

Rational design and synthesis of transition metal sulfide complex nanostructures are significant for achieving desired materials properties for a variety of applications. Herein, we synthesized nickel sulfide (NiS) nanostructures using laser irradiation in an aqueous solution and under the ambient condition. Crystalline nanostructures with high phase-purity were achieved without further calcination. Other transition metal sulfides, such as CuS and ZnS, were also successfully synthesized using the laser irradiation approach, suggesting the practical application of this method. The hydrogen evolution reaction (HER) measurements show the NiS nanostructures synthesized for 4h (NiS-4h) not only exhibited competitive overpotential (−159 mV vs. RHE at 10 A/g), lower Tafel slope (218 mV/dec) but also delivered long-term stability (14 A/g at −250 mV vs. RHE for 12h). Moreover, the NiS-4h functioned as the electrode for supercapacitor with excellent specific capacitance (3761 F/g at 10 mV/s), reversibility and rate capability (1152 F/g at 100 mV/s). These superb electrochemical performances are attributed to the remarkable value of electrochemically active surface area (ECSA) and synergistic effect of Ni3+OOH for HER electrocatalytic activities, while the co-existence of Ni2+ and Ni3+ facilitates the rich redox reactions of NiS for supercapacitor.

Rational design and synthesis of transition metal sulfide complex nanostructures are significant for achieving desired materials properties for a variety of applications. Herein, we synthesized nickel sulfide (NiS) nanostructures using laser irradiation in an aqueous solution and under the ambient condition. Crystalline nanostructures with high phase-purity were achieved without further calcination. Other transition metal sulfides, such as CuS and ZnS, were also successfully synthesized using the laser irradiation approach, suggesting the practical application of this method. The hydrogen evolution reaction (HER) measurements show the NiS nanostructures synthesized for 4h (NiS-4h) not only exhibited competitive overpotential (−159 mV vs. RHE at 10 A/g), lower Tafel slope (218 mV/dec) but also delivered long-term stability (14 A/g at −250 mV vs. RHE for 12h). Moreover, the NiS-4h functioned as the electrode for supercapacitor with excellent specific capacitance (3761 F/g at 10 mV/s), reversibility and rate capability (1152 F/g at 100 mV/s). These superb electrochemical performances are attributed to the remarkable value of electrochemically active surface area (ECSA) and synergistic effect of Ni3+OOH for HER electrocatalytic activities, while the co-existence of Ni2+ and Ni3+ facilitates the rich redox reactions of NiS for supercapacitor.

Rechargeable battery technology has been one of the most exciting advancements in science and technology in the last several decades. Even though lithium ion battery technology has achieved great success in many fields, its wide deployment for large-scale energy storage is very questionable because of the limited resources of lithium. Therefore, alternative rechargeable battery technologies based on abundant elements need to be developed for sustainable electrochemical energy storage. Among all the potential candidates, battery systems based on aluminum as anode is particularly promising. In this thesis, we are dedicated to develop novel rechargeable Al-ion battery prototypes mainly focusing on cathode materials. Our achievement of discovering transition metal sulfides as promising cathode materials for rechargeable Al-ion batteries is pioneering. We first proposed Chevrel Phase Mo6S8 as intercalation-type cathode material for rechargeable Al-ion battery using ionic liquid electrolyte. We investigated the electrochemical properties as well as compositional properties of Chevrel Phase Mo6S8 as a cathode material. We believe it is the first reported intercalation-type rechargeable Al-ion battery prototype. We went further to probe the detailed intercalation process of Al3+ in Chevrel phase Mo6S8. High quality powder XRD data along with Rietveld refinements give a clear picture of the aluminum intercalation induced phase transition process of Mo6S8. High resolution TEM further provided strong evidence of Al3+ intercalation and phase transition of Mo6S8. In light of the information gained from the Al3+ intercalation and deintercalation process, constant-current-constant-voltage charge and galvanostatic discharge technique was used to improve the cycling performance of Mo6S8. A 50% capacity increase was obtained with a high current density of 40 mA g-1. At last, we extended our cathode materials screening on other transition metal sulfides base on the previous results. W

Rechargeable battery technology has been one of the most exciting advancements in science and technology in the last several decades. Even though lithium ion battery technology has achieved great success in many fields, its wide deployment for large-scale energy storage is very questionable because of the limited resources of lithium. Therefore, alternative rechargeable battery technologies based on abundant elements need to be developed for sustainable electrochemical energy storage. Among all the potential candidates, battery systems based on aluminum as anode is particularly promising. In this thesis, we are dedicated to develop novel rechargeable Al-ion battery prototypes mainly focusing on cathode materials. Our achievement of discovering transition metal sulfides as promising cathode materials for rechargeable Al-ion batteries is pioneering. We first proposed Chevrel Phase Mo6S8 as intercalation-type cathode material for rechargeable Al-ion battery using ionic liquid electrolyte. We investigated the electrochemical properties as well as compositional properties of Chevrel Phase Mo6S8 as a cathode material. We believe it is the first reported intercalation-type rechargeable Al-ion battery prototype. We went further to probe the detailed intercalation process of Al3+ in Chevrel phase Mo6S8. High quality powder XRD data along with Rietveld refinements give a clear picture of the aluminum intercalation induced phase transition process of Mo6S8. High resolution TEM further provided strong evidence of Al3+ intercalation and phase transition of Mo6S8. In light of the information gained from the Al3+ intercalation and deintercalation process, constant-current-constant-voltage charge and galvanostatic discharge technique was used to improve the cycling performance of Mo6S8. A 50% capacity increase was obtained with a high current density of 40 mA g-1. At last, we extended our cathode materials screening on other transition metal sulfides base on the previous results. W

The transition metal sulfide Fe4Ti3S8 with 7:8 composition has been synthesized and studied by using X-ray diffraction, magnetization and electrical resistivity measurements. This compound exhibits a monoclinic crystal lattice (space group I12/m1). The substitution of Ti for Fe in Fe7S8 is found to result in a lowering of the Curie temperature (TC ≈ 205 K), in a larger value of the coercive field (Hc ∼ 9 kOe at low temperatures) and in a substantial growth of the resultant magnetic moment per formula unit (μFU) in comparison with Fe7S8. An enhanced value of μFU is attributed to the preferential substitution of Ti in alternating cation layers. From the paramagnetic susceptibility measured within temperature interval (250-350) K, a reduced value of the effective moment per iron (μFe ∼ 2.4μB) was determined. The electrical resistivity of Fe4Ti3S8 shows a non-metallic behavior and is affected by magnetic ordering. © 2013 Elsevier Masson SAS. All rights reserved.

The transition metal sulfide Fe4Ti3S8 with 7:8 composition has been synthesized and studied by using X-ray diffraction, magnetization and electrical resistivity measurements. This compound exhibits a monoclinic crystal lattice (space group I12/m1). The substitution of Ti for Fe in Fe7S8 is found to result in a lowering of the Curie temperature (TC ≈ 205 K), in a larger value of the coercive field (Hc ∼ 9 kOe at low temperatures) and in a substantial growth of the resultant magnetic moment per formula unit (μFU) in comparison with Fe7S8. An enhanced value of μFU is attributed to the preferential substitution of Ti in alternating cation layers. From the paramagnetic susceptibility measured within temperature interval (250-350) K, a reduced value of the effective moment per iron (μFe ∼ 2.4μB) was determined. The electrical resistivity of Fe4Ti3S8 shows a non-metallic behavior and is affected by magnetic ordering. © 2013 Elsevier Masson SAS. All rights reserved.